Insulated coil for arrangement in a slot in the stator or rotor of an electric machine

ABSTRACT

A coil positioned in the slot of a stator or rotor of an electric machine is formed of a bundle of conductors arranged close together and insulated from each other and from the slot. The conductor insulation consists of corona resistant insulating material running longitudinally of the conductor and surrounding the parts of the conductors facing the main insulation which surrounds the bundle of conductors and also overlies at least those part of the faces of the conductors which face the adjacent conductors which parts are situated nearest the tape facing the main insulation. The tape is formed of a linear unbranched or branched polymer having such thermal stability that the dielectric strength of a film of the polymer 50 microns in thickness, after ageing in air at 155*C for 25,000 hours, is at least 50 percent of the dielectric strength before the ageing and of mica in the form of small flakes.

United States Patent 1 Andersson et a1.

[ 1 INSULATED COIL FOR ARRANGEMENT IN A SLOT IN THE STATOR OR ROTOR OF AN ELECTRIC MACHINE Inventors: AndersR. Andersson; Anders Bjorklund; Lars-Goran Virsberg, all ofVasteras, Sweden Assignee: Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden Foreign Application Priority Data Dec. 2, 1971 Sweden 15448 Filed: July 10, 1972' Appl. No.: 270,439

US. Cl 310/208, 156/54, 161/163, 174/120 SR, 336/206 Int. Cl. H011 27/32 Field of Search 336/206; 161/163, 161/171;174/120,l20 SR,121,121 ,SR;

References Cited UNITED STATES PATENTS Roos 336/206 Pentland 336/206 X Purvis 161/163 X Berberich et al. 174/121 SR 1 Nov. 27, 1973 3,523,061 8/1970 Purvis 161/163 X 2,917,570 12/1959 Wolff et a1. 174/120 SR 753,461 3/1904 Anderson 336/206 3,019,286 8 H1962 Andersson et al. 174/121 2,707,204 4/1955 Richardson et a1; 174/120 2,970,936 2/1961 Richardson; 174/120 FOREIGN PATENTS OR APPLICATIONS [/1952 Great Britain 161/163 1 Primary Examin er-Thomas J. Kozma Att0rneyJennings Bailey, Jr.

57 ABSTRACT A coil positioned in the slot of a stator or rotor of an electric machine is formed of a bundle of conductors arranged close together and insulated from each other and from the slot. The conductor insulation consists of corona resistant insulating material running longitudinally of the conductor and surrounding the parts of the conductors facing the main insulation which surrounds the bundle of conductors and also overlies at least those part of the faces of the conductors which face the adjacent conductors which parts are situated nearest the tape facing the main insulation. The tape is formed of a linear unbranched or branched polymer having such thermal stability that the dielectric strength of a film of the polymer microns in thickness, after ageing in air at C for 25,000 hours, is

at least 50 percent of the dielectric strength before the ageing and of mica in the form of small flakes.

11 Claims, 7 Drawing Figures 8 PAIENIED WZ 3,775 62 Fig. Fig 2 #INSULATED:COIUFOR ARRANGEMENT lNA )sSLOT IN THE STATOR= OR :ROTORIOFAN i EEECTRIC M'ACHINE BACKGROUND OF'TI-IE INVENTIQN L-Field of the Invention The invention relates toaninsulated coil'for arrangewment in a slot in thestator or rotor'ofz an electric machine. V

'2. The Prior Art I Acoil which isarranged in a slot inthe'. stator or rotor expensiveand theinsulation is easily'damaged byfolds andcracks arising during the bendingiprocess or duri'ng other handling of the conductor while shaping thecoil.

f anzelectric machine oftencomprises a 'bundle ofseveral conductors arranged close together. iandrprovided with a conductor insulation to'insulate.the conductors jfrom each other. The bundlemay consist-ofturns of a 'single insulated conductor. It.;-mayalso consist of sevreralseparate insulatedconductors (strands) which are electrically connected,.usually parallel-connected, to

:each other .at the ends.

The conductonbundle :is. surrounded by a .main' insu- -lati'on to. insulate the bundle from the: machine 'slot which has earth potential. The "main insulation usually includes materialwhich hasstrongresistanceto corona attack, forv example :mica or siliconerubber and there .are :normallyimpregnatingand pressing operations dur ing 'itstapplication around "the 'bundle of conductors so that an insulation is obtained having no voids and 'tightly. surrounding the .rcondu'ctor bundle. Inpractice it has'been impossibleto ensure the manufacture of coil insulations which are permanently Efree "from inner spaces where corona may occur. T-he occurrence of small:gas-filled voids even during the manufacture of the insulation cannot be completely avoided. Furthersulation there is great riskof its .being destroyed and weakened so that short-circuiting between adjacent turns of a single insulated conductor or -short-circuiting between strands may occur. In orderto avoid this risk a conductor insulation is aimed at which has strong resistance to corona attack. A conventional embodiment of conductor insulation which also provides corona protection is one in which the-conductor is'wrapped with mica tape consisting of: micaflakesattached usually with a resinous binder to a carrier material of glass cloth, for example. With respect to the occurrence of temporary steep over-voltages, for example connection overvoltages, which may cause great stress between adjacent turns in a coil, the requirements of the electrical strength of the conductor insulation are extremely high seen in relation to the stress'occurringduring normal operation. .A normal rule is that insulationbetween Ladjacent turns shall be dimensioned towithstand a voltage equal to the rated voltage of the machine. In order to achieve sufficient electric strength with conductor insulation of conventional mica tape, a. relatively thick insulation is requiredwhich is effected by winding on several layers of tape. This conductor insulation has Anothertype of conductor insulation where these disadvantages are less pronounced isan embodiment in which the insulation of the conductor consists ofalayer of enamel of a type normally used forenamelling conducting wire and a wrapping of mica tape appliedoutside this enamel as corona protection. Because of the 7 high electric strength of .the wire enamel layer, the total thickness of the insulation can in this case be kept lowerthan when the conductor is only wrapped with mica tape. In order effectively toprotect the enamel layer against corona and at the sametime contribute somewhat to the electric strength, the outer'wrapping -of tape must becarried outwith overlapbetwe'en adja- 'cent turns ofthetape. In this casealso thetape wr'apping causes problems when the conductors'are being be'nt. If the tape is loosely bound to-the enamel layer below it, it has a tendency to slide apartwh'en 'thecon- "ductor is bent. If,on the other hand, the tapeis firmly attached to the layer of enamel, cracks may easily -occur during the bending.

Itis'extremely desirable for the insulation andcorona protection around theconductor to be as'thin and take "as little space as possible. Inthis way more-conductor material can be incorporated in the-coil and a greater 'effectis achieved for a given machine'dimension or for to VDE 0345/ l .65 S 17. According to the present inseveral disadvantages. It requires considerablespace, is

a given effect the machine dimension'can be reduced.

SUMMARY OF THE INVENTION According to the present invention it hasbeen discovered that itis possible to use considerably thinner conductor insulation, which at the same time forms the corona protection, in coils of the type 'describedthan in previously known coils, and at the same time'achiev'e equally good electrical qualities in the coils.

The present invention relates to a coil to be arranged in the slot of a stator or rotor in 'an electric machine, comprising a bundle of several conductors arranged close together and provided with conductor insulation being corona resistant and containing micato insulate the conductors from each other and a main insulation surrounding the bundle to insulate the bundle from the machine slot, the conductors having sides facing each other and at least one side facing the main insulation,

characterised in that each side facing the main "insulation and at least the parts located nearest each such side, of the sides facing adjacent conductors, is surrounded by a tape of insulating materialacting as conductor insulation running in the longitudinal direction of the conductor, at least one layer of the-tapebeing arranged between adjacent conductors and that the tape is built up of a linear unbranched or branched'polyrner having such thermal stability that the I dielectric strength of a film of the polymer 50 microns-in thickness, after ageing in air at C for 25,000 =hours,-is at least 50 percent of the dielectric strength before the ageing and of mica in the form of small flakes.

The dielectric strength can be determined according vention it is possible in the area where the stressesare greatest, i.e. between the sides of the conductors which are facing the main insulation and the machine slot, to achieve effective corona protection using only one layer of the tap, whereas normally at leasttwo layers must be used since in the latter case the tape'must be form a mat.

applied with overlap between adjacent turns. According to the invention, therefore, the outer dimensions of the conductor bundleare considerably less than for acorresponding conductor bundle in which an identical tape of insulating material has been wound aroundthe conductor with overlap in conventional manner. The fact that joints or gaps occur according to the present invention between the tape along thesides of the con-' ductors in the coildescribed which are' facing each other has not proved to be any disadvantage. This is explained because thestresses between the conductors are low during normal operation and any over-voltages which arise are very brief, as mentioned previously.

The present invention is especially intended for use in electric machines for operating voltages above 3 kV in view of the corona problems arising in such machines.

The conductor bundle may consist of several turns of a single insulated conductor which is bent to give the final shape of the bundle. It may also consist of several separate insulated conductors, often called strands, which are electrically joined to each other, usually parallel-connected, at their ends.

The main insulation may be designed in various well known ways and its construction does not form any part of the present invention. It may, for example, consist of a bandage of mica tape or mica sheet. These may be made of large mica flakes of conventional type which, for example with a varnish binder such as an alkyd resin or a thin thermoplastic film, are attached to a carrier material of paper, glass cloth or the like. The mica material may also consist of self-carrying tape or sheets of small mica flakes overlapping each other, manufactured by splitting ordinary micarln this case the mica material is usually attached to a carrier material as well.

A binder may be applied on saidv mica tapes or arcs which, when the material is later applied around the conductor bundle, binds the various layers of the conductor insulation to each other. However, such a binder may "even be supplied after the insulating material has been applied around the conductor bundle by means of an impregnation. As examples of suitable binder for the turns in the wrapping may be mentioned solventless resins such as epoxy resins and unsaturated polyester resins.

The main insulation may alternatively consist of silicon rubber, for example, applied in the form of a strip or paste around the bundle of conductors.

The conductor insulation which is the object of the present invention as mentioned earlier, is built up of a linear unbranched or branched polymer having special properties and of mica in the form of small flakes.

According to one embodiment of the invention the tape of insulating material is built up of a coherent film of the polymer with the mica flakes homogenously embedded as filler.

According to another embodiment of the invention the tape of insulating material is built up of a coherent film of the polymer with the mica flakes arranged layer on one or both surfaces of the film.

According to another embodiment of the invention the tape of insulating material is built up of a mixture of mica flakes and short fibres of the polymer which As examples of suitable polymers may be mentioned polyamide imides, polyamides, polyimides, polymers 'asa The mica flakes suitably have a size less than 2 mm".

If the mica flakes are embedded in the polymer, the content of mica is preferably 25 percent of the total weight of mica and polymer.

If the mica flakes are arranged on the surface of a polymer film, they are suitably attached to this and to each other by a thin layer of a resinous binder so that a layer of falkes overlapping each other is obtained. The layer, or each layer of mica flakes, then has a thickness of preferably 1 50 microns. I

If the tape of corona-resistant material is formed of a mixture of small mica flakes and short fibres of the polymer the percentage of mica is preferably 25 75 percent of the total weight of mica and fibres. Such a tape can be manufactured by mixing fibres and mica flakes to a pulp and then shaping the mixture of components to a sheet of material in accordance with methods similar to those used in the manufacture of paper. The fibres have suitably a length of 0.5 25, preferably 1 10 mm and a thickness of l 50, preferably 5 25 microns. The conductor insulation, i.e. the polymer with incorporated mica flakes or layers of mica flakes on the surface has a thickness of suitably l0 200 microns, preferably 25 microns.

The conductor insulation is suitably attached to the conductor with a poreless resinous binder having high elongation at rupture and good stiffness. As examples of suitable binders may be mentioned flexible polyurethane resins, epoxy resins modified with polyamides,.

polyester resins modified with isocyanates, epoxy resins modified with polyurethane resins and certain types of rubber. Such resinous binders are also suitable as bind ers for the previously mentioned attachement of mica to the surface of a polymer film.

If the conductor has two sides facing the main insula- BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail by means of a number of examples with reference to the accompanying drawings in which FIG. 1 shows in perspective a conductor according to the invention provided with conductor insulation prior to bending, t

FIG. 2 the same conductor in cross section on a larger scale,

FIG. 3 a side view of a coil produced by bending the conductor according to FIGS. 1 and 2 and provided with main insulation,

FIG. 4 the same coil in cross section and FIGS. 5 7 alternative embodiments of the conductor according to FIGS. 1 and 2 shown in cross section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE I averagemolecular Weight of about 2500, 24 parts by weight of an epoxy resin consisting of diglycidyl. ether of bisphenol A having an epoxy equivalent of 190 (for exampleDER-33l from Dow Chemical, USA) and 2 parts by weight dicyandiamide. This resin, dissolved-to a 25 percent solutionin equal'parts methanolandtrichloroethylene, is applied as a thin layer on'one side of afilm 50 micronsthick and 25 mm wide, consistingof equal parts by weight polyimide and small mica flakes,

Y the mica flakes being uniformly distributed in the volumeof the'film; When the solvent had evaporated the film was applied as conductor insulation 11, with the resinous binder 12 facing inwards along the conductor 3 snvsasgiv which is 3 X 8 mm so that the edges 13 and 14 overlapped each other in the longitudinal direction of the conductor. The product thus formed" was then placed in a mould andsubjected to heating to 200C for. some minute, so that the resinous binder melted andbecame :cured.

I The conductor provided witlrinsulating material l l was beiit to a coil having severalturns of the conductor lying 'close'l to each otherias shown in FIGS. 3

' and'4; respectively. (These figures show the 'coil when the. main insulation has been applied). The said turns of the conductor lie close together and form the conductor bundle of the coil. As is clear from FIG. 3 the conductor is b'entyso that -thecoil hasistra'ight parts 15' and 16 intended to'be placed in the machine slots and curvedendparts 17' and 18 which come outside the slots. The terminals of the coil are designated 19 and The conductor bundle is then wrapped in -turns with, for example, half overlap, with a mm wide mica tape consisting; of 'a 0.09 mm thick self-carrying layer of 'small mica flakes overlapping each other fixed to a 0.04 mm thick woven' glass tape with 01006 mm thick polyethylene glycolterephthzilateffoil. Such an insulating tape is described in British patent No. 939,747. When the bundle has been provided with a wrapping; 21 of, for example, thirty layersof mica tape one on top of the other, the winding is dried first at a pressure of 0.1mm

I-Igand a temperature of 40C, after which an impregnatingresin is supplied at the samepressure. When all theiimpregnating resin has been'supplied; the pressure is raised to, for example 25 kg/cm The impregnating resin may consist of an epoxyresin consisting, forexample of 85 parts Araldit-F, l00parts Curing agent 905 (both fromCiba AG,-,Switzerland) and l-S'parts phenyl glycidyl ether. So that the impregnating resin shall not penetrate out of theinsulation during the subsequent curing process the impregnated condulstor bundle with thewrapping of mica tape may be surrounded'with a sealing tape of polytetrafiuoroethylene or the like. The coil is then placed in amoulding toolto cure the impregnating. resin. Curing-takes placeat a temperature of about.l60C for 4 to 6 hours. The coil.

is applied with the straightslot-parts in the machine slot 22 asshown in FIG. 4.

EXAMPLE 2 p A coil was manufactured in the manner indicated in Example 1, with the difference that as resinous binder to attach the polymer film to the conductor-a semicured polyimide-resin was used, applied on one or both I sides of the film. The polymer filmwas attached to the conductor by heatingto 320C for 3 minutes.

. EXAMPLE-'3 EXAMPLE 4 A coil was manufactured in the manner indicated in. Example I with the difference that the mica-coated polymer film described in Example 3 is used and that this film is attached to the conductor with a resinous binder consisting of a polyamide-modified epoxy resin- (for example AF-42 from Minnesota Mining and Manufacturing Company). The resin is then dissolved in asolvent and coated in a thin layer on the conductor and on one surface of the film at the overlapping point:

"When the solvent has evaporated the product thus formed is placed in a mould and subjected to heating to 165C. Furthermore, as impregnating-resin in-themaininsulation an unsaturated polyester resin'is used manufactured from adipic acid (I 1 mole ),phthalic acid anhydride (1 1 mole maleic acid anhydride (23" mole and ethylene glycol (55'mole and provided with diallylphthalate in such a quantity that-thediallyl phthalate comprises 40 percent of the total weight of polyester resin and diallyl phthalate and also with benzoyl peroxide in-a quantity correspondingto'l percent of the weight of the mixture. The polyester resin itself is'manufactured by reaction of a mixture of said acids and the alcohol in inert atmosphere by increasing. the

temperature to 220C and maintaining this temperature until the acid number of the reaction product is about 30. The coil is dired prior to impregnation at'a pressure of less than lmm Hg and'the resin is cured; after the impregnation at a temperature of C for 1 hour.

EXAM PLE 5 A coil is manufactured in the manner indicatedin Example l with the difference that instead of the tape. of

insulating material described there a tape 23 according. to FIG. 5 of the same insulating material and thesame thickness but havinga width of 20 mm is used. A gap? 24 thus appears between the two edges 25 and '26 o'f the tape. Under the tape on the side where the'gap'islocated is a polymer film 27, for example ofpolyethylene glycol terephthalate having a thickness of "20 microns, this film being attached to the conductorand tothe film 23 with the resinous binder 12. When the'conductors'are placed in the slot, this-is done so that one side 28*with a gap 24 of one conductoris turned towards'a' side 29'without a gap in the adjacent conductor. Of" course the width of the tape 23 of insulatingmaterialf can be chosen so thatthe gap will be zero, i.e. the'edges 25 and 26 meet.

EXAMPLE 6 In a coil with parallel-connected strands, the stresses between these are low. FIG. 6 shows a suitable "insulation for such a strand. Thetape of insulating material 30 has a thickness of 75 microns and a width of 20 mm. The dimensions of the conductor are 3 X 8 mm as before. The tape consists of a mat built up of a mixture of equal parts by weight of small micaflakes and short fibres of an aromaticpolyamide (for example NOMEX I coated in a thin layer on the conductor. When the solvent has evaporated the tape 30 is attached to the conductor by placing the conductor provided with the tape in a mould tool and subjecting them to-heating up to 165C for 30 minutes. A gap 33 is formed between the edges 31 and 32 of the tape, which possibly may be zero in width. The conductors are placed in the slot 22 in such a way that a side 34 with gap 33 of one conductor abuts a side 35 without gap of an adjacent conductor.

The main insulation 25 may be of the same-type as that in Example 1.

EXAMPLE. 7

A coil is manufactured in the manner indicated in Example 6 with the difference that instead of-a tape 30, two tapes 36 and 37 of the same insulating material are used, each being 9 mm wide. Each tape 36, 37, is applied as shown in FIG. 7, displaced in relation to a plane through the middle of the short sides of the conductor so that the gaps 38 and 39 between the edges of the tape will be situated on opposite sides of a plane through the middle of the long sides of the conductor. In this way the gaps in two adjacent conductors will not cover each other at any point if they are stacked one on top of the other with a side 40 of one conductor abutting a side 41 of an adjacent conductor. This ensures that there is at least one layer of the insulating tap arranged between adjacent conductorsj The gaps 38 and 39 may be zero, i.e. the two edges may meet each other on both sides of the conductor, but on opposite sides of a plane through the middle of the long sides.

. "Of course mica tape having large mica flakes or other commercially available completely polymerisable resins, such as for example asphalt, can be used for the main insulation in the coils described in the Examples. Although many unsaturated polyester resins and epoxy resins suitable for impregnating electric insulations are known, a few more examples of such resins will be mentioned. For example, an impregnating resin consisting of 60 parts by weight of a reaction product of 3 moles maleic acid anhydride, 1 mole adipic acid, 4.4 moles ethylene glycol manufactured accordingto the process described earlier, with an acid number of 30,

and of 40 parts by weight diallylphthalate containing- 0.75 benzoyl peroxide may be used, or an impregnating resin consisting of 70 parts by weight of a reaction product of 1 mole fumaric acid, 1 mole 'phthalic'acid and 2.2 moles propylene glycol reacted to'an acid number of 25 and of 30 parts by weight monostyrene con-.

taining 0.5 benzoyl peroxide. Amongst' suitable epoxy resins may be mentioned a product consisting of 100 parts by weight Epon 828 (shell Chemical Co.) and of 65 parts hexahydrophthalic acid anhydride, or

a product consisting of 100 parts by weight Dow 331 8 (Dow Chemical Co.) and 65 parts by weight tetrahydrophthalic acid anhydride. We claim: 1

l. Coil to be arranged in the slot of a stator or rotor in an electric machine, comprising a bundle of several conductors arranged close togetherand providedwith conductoririsulation to insulate the conductors from each other and amain insulation surrounding the bundle to insulate the bundle from the machine slot, the conductors having sides facing each other and at least one side facing the main insulation, said conductor having conductor insulation comprising tape means of insulating material running in the longitudinal direction of the conductor, the tape means surrounding said one side facing the main insulation and at least those parts of the sides facing adjacent conductors which are situated nearest said one side, at least one layer of the tape means being positioned between adjacent conductors, the tape means comprising a linear unbranched or branched polymer having such thermal stability that the dielectric strength of a film of the polymer 50. mi-

crons in thickness, after ageing in air at C for 25,000 hours, is at least 50 percent of the dielectric strength before the ageing and mica in .the form of small flakes.

2. Coil according to claim 1, in which the tape means of insulating material is built upof a coherentfilm of the polymer with the mica flakes embedded as filler.

3. A coil according to claim. 1, in which the tape means of insulating material is built up of a coherent film of the polymer with the mica flakes arranged as a layer on at least one side of the film.

4. Coil according to claim 1, in which the tapemeans of insulating material is built up of a mixture of mica flakes and short fibres of the polymer.

5. Coil according to claim 1, in which thetapemeans of insulating material is wider than the circumference of the conductor and is folded around the conductor with the edges overlapping each other.

6. Coil according to claim 5, in which said tape meansv comprises a single tape.

7. Coil according to claim 4, in which the tape means comprises a tape of insulating material surrounding only one side of the conductor facing the main insulation and at least those parts of the sides of the conductor facing adjacent conductors which are situated nearest said one side, another tape of insulating material surrounding one side of the conductor positioned opposite to said one side facing the main insulation and at least those parts of the sides of the conductor facing adjacent conductors which are situated nearest to said side positioned opposite to said one side facing the main insulation. I v

8. Coil according to claim 6, in which said two tapes overlie parts only of the sides facing adjacent conductors and the gap between the edges of the two tapes on one side of a first conductor facing an adjacent conductor is displaced laterally in relation to'the gap between the edges of the two tapes on the side of the adjacent conductor facing the first conductor. so that at least one layer of tape is present between the conductors.

9. Coil according to claim 1, in which the tape means comprises a single tape surrounding one side of the conductor facing the main insulation, one side of the conductor positioned opposite to said one side facing the main insulation, one side of the conductor facing an adjacent conductor and at least those parts of another achieve good electric strength.

11. Coil according to claim 1, in which the polymer consists essentially of a substance selected from the group consisting of polyamide imides, polyamides, polyimides, and polymers based on polyhydantoin, polyethylene glycol terephthalate, polycarbonate and polysulphon.

Disclaimer 3,7 7 5,628.Anders R. Andev'sson, Anders B joflclund, and Laws-Goran Virsbea g, Vasteras, Sweden. INSULATED COIL FOR ARRANGEMENT IN A SLOT IN THE STATOR OR ROTOR OF AN ELECTRIC MACHINE. Patent dated Nov. 27, 1973. Disclaimer filed Mar. 26, 1974, by the assignee, Allmmma Swenslca Elelctm'slca Alctiebolaget. Hereby disclaims the term of the patent subsequent to Mar. 27 1990.

[Ofiicial Gazette July 1, 1975.]

Disclaimer 3,77 5 ,628.Ande1=s R. Andewsson, Andews B jowlclund, and Lars-Gown Virsbewg, Vasteras, Sweden. INSULATED COIL FOR ARRANGEMENT IN A SLOT IN THE STATOR OR ROTOR OF AN ELECTRIC MACHINE. Patent dated Nov. 27, 1973. Disclaimer filed Mar. 26, 1974, by the assignee, Allmanna Svenslca Elekm'sica Alatiebolaget. Hereby disclaims the term of the patent subsequent to Mar. 27, 1990.

[Oficz'al Gazette July 1, 1975.] 

1. Coil to be arranged in the slot of a stator or rotor in an electric machine, comprising a bundle of several conductors arranged close together and provided with conductor insulation to insulate the conductors from each other and a main insulation surrounding the bundle to insulate the bundle from the machine slot, the conductors having sides facing each other and at least one side facing the main insulation, said conductor having conductor insulation comprising tape means of insulating material running in the longitudinal direction of the conductor, the tape means surrounding said one side facing the main insulation and at least those parts of the sides facing adjacent conductors which are situated nearest said one side, at least one layer of the tape means being positioned between adjacent conductors, the tape means comprising a linear unbranched or branched polymer having such thermal stability that the dielectric strength of a film of the polymer 50 microns in thickness, after ageing in air at 155*C for 25,000 hours, is at least 50 percent of the dielectric strength before the ageing and mica in the form of small flakes.
 2. Coil according to claim 1, in which the tape means of insulating material is built up of a coherent film of the polymer with the mica flakes embedded as filler.
 3. A coil according to claim 1, in which the tape means of insulating material is built up of a coherent film of the polymer with the mica flakes arranged as a layer on at least one side of the film.
 4. Coil according to claim 1, in which the tape means of insulating material is built up of a mixture of mica flakes and short fibres of the polymer.
 5. Coil according to claim 1, in which the tape means of insulating material is wider than the circumference of the conductor and is folded around the conductor with the edges overlapping each other.
 6. Coil according to claim 5, in which said tape means comprises a single tape.
 7. Coil accordiNg to claim 4, in which the tape means comprises a tape of insulating material surrounding only one side of the conductor facing the main insulation and at least those parts of the sides of the conductor facing adjacent conductors which are situated nearest said one side, another tape of insulating material surrounding one side of the conductor positioned opposite to said one side facing the main insulation and at least those parts of the sides of the conductor facing adjacent conductors which are situated nearest to said side positioned opposite to said one side facing the main insulation.
 8. Coil according to claim 6, in which said two tapes overlie parts only of the sides facing adjacent conductors and the gap between the edges of the two tapes on one side of a first conductor facing an adjacent conductor is displaced laterally in relation to the gap between the edges of the two tapes on the side of the adjacent conductor facing the first conductor so that at least one layer of tape is present between the conductors.
 9. Coil according to claim 1, in which the tape means comprises a single tape surrounding one side of the conductor facing the main insulation, one side of the conductor positioned opposite to said one side facing the main insulation, one side of the conductor facing an adjacent conductor and at least those parts of another side of the conductor facing another adjacent conductor which are situated nearest said one side of the conductor facing the main insulation and said one side positioned opposite to said side facing the main insulation.
 10. Coil according to claim 1, in which the main insulation comprises a wrapping of a tape or sheet shaped insulating material with mica as component in order to achieve good electric strength.
 11. Coil according to claim 1, in which the polymer consists essentially of a substance selected from the group consisting of polyamide imides, polyamides, polyimides, and polymers based on polyhydantoin, polyethylene glycol terephthalate, polycarbonate and polysulphon. 